The goal of the Program Project Grant is to discover the manner in which coordinated movement results from interactions between the musculoskeletal system and neural circuits in the spinal cord. The purpose of this project is to investigate the role of proprioceptive pathways in mediating intermuscular and interjoint coordination during multi-directional movements. Experiments will be performed on freely moving cats and decerebrate preparations. The patterns of recruitment of key output elements which act at the ankle, including both whole muscles and compartments, will be observed in normal animals during sagittal plane locomotion and turning movements. In terminal experiments, the organization of proprioceptive pathways interconnecting these key output elements will be investigated in the decerebrate state. Proprioceptive feedback from these output elements, but not motor output, will be rendered ineffective by the process of self-reinnervation, and the effects of this procedure on coordination and reflex organization evaluated. The importance of feedback in coordinating postural muscles in the cat hindlimb will be tested by severing the tendons of selected weight-bearing muscles. If intact muscles with similar actions but different patterns of usage assume the activity patterns of the tenotomized muscles, then the hypothesis will be supported if strong intermuscular reflexes develop from and to the intact muscles. The experiments proposed here and elsewhere in this Program Project Grant are intended to help revise and extend current models of spinal cord function in motor control. These advances will lead to more comprehensive diagnoses and new treatments for motor disorders.